Spaghetti or rotelle?
This is not a culinary question – it’s a materials science one, and answering it could bolster the battery life of future smartphones.
Right now, organic LEDs are made with repeated molecular units that look like a spaghetti noodle. The linear shape means that light can move in just one direction, allowing just 20 percent of the light to escape the LED and leaving the other 80 percent trapped inside.
And LEDs can do better, according to a new paper published in Nature Chemistry on Sunday that describes a round organic molecule that could make LEDs, already at the forefront of energy efficient, high-tech lighting, even more efficient.
The new shape for the cooked-up molecule still takes its cues from pasta. But, instead of spaghetti, this time it’s rotelle, also known as wagon-wheel pasta. This six nanometer wide, spoke-wheeled molecule can emit light in all directions, significantly reducing the amount of light wasted in LEDs, the authors say.
“OLEDs are yet efficient,” says Sigurd Höger, a researcher involved in synthesizing the rotelle-shaped molecule at the University of Bonn and an author on the paper. “However, there are some problems with the light that is trapped inside the device.”
“And maybe we can make them more efficient,” he said.
LEDs, or light emitting diodes, have found out a plum spot in the lighting market in recent years. They use just 25 percent of the energy that an incandescent bulb requires and last about 25 times longer. LEDs illuminate households, make street lamps glow, backlight smartphone screens, and ring in the New Year on the ball dropped in Times Square. They have, in short, come light years from the unappealing red spots that first blinked onto the market in the 1960s.
LEDs have also acquired a sibling: Organic LEDs. Often called OLEDs, these lights use organic polymers, not silicon semiconductors, and are more pliable than conventional LEDs. Still less common relative to their inorganic counterparts, OLEDs have all the glitz and gloss of the tech of the future, appearing in Samsung’s Galaxy smartphone series and in its line of ultra-thin (and ultra-expensive) televisions and laptops, as well as in crisp and bright products from Sony and LG.
But efforts to make OLEDs even more efficient have bumped against a prohibitive factor: single polarization. Light waves oscillate, and the direction in which they oscillate is called a polarization. In current OLEDs, an electric current running through the spaghetti-like polymer is limited to moving in one direction. In other words, it has one polarization.
This results in just 20 percent of the light actually getting out of the OLED, an inefficient use of energy that means that even the latest models of smartphones have, at best, about two days battery life.
But a wagon-wheel molecule – what the researchers call a “π-conjugated spoked-wheel macrocycle” – has an advantage over the spaghetti-design: it is symmetrical. This round shape emits light in not just one direction, but in any direction at random – that is, light is not restricted to a single polarization. The scenario can be imagined as attempting to balance a pencil straight up on its tip: let go, and the pencil will fall over in a random direction each time, the authors said.
Using the round molecules will reduce the light waste from about 80 percent to 50 or 60 percent, a feat that could keep smartphones juiced up for longer, the authors reported.
“If the light output is more efficient, less light has to be generated and that saves the battery,” says Dr. Höger.
Rotelle-shaped molecules will require extensive research before they are menu-ready, says Höger, as these new molecules were synthesized at a cost prohibitive to commercial purposes.
A rotelle-shaped molecule “is something for the future,” he said. For now, we’ll have to make do with good, ol’ spaghetti.